In electostatographic imaging and recording processes such as electrophotographic reproduction, an electrostatic latent image is formed on a primary image-forming member such as a dielectric surface and is developed with a thermoplastic toner powder to form a visible image. The visible thermoplastic toner powder image is thereafter transferred to a receiver, e.g., a sheet of paper or plastic, and the visible thermoplastic toner powder image is subsequently fused to the receiver in a fusing station using heat or pressure, or both heat and pressure. The fuser station can include a roller, belt or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver.
The fusing operation with a roller fuser commonly comprises passing the image-bearing receiver between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the fusing nip, at least one of the rollers typically has a compliant or conformable layer on its surface. Heat is transferred from at least one of the rollers to the visible thermoplastic toner powder in the fusing nip, causing the toner powder to partially melt and attach to the receiver. In the case where the fuser member is a heated mller, a resilient compliant layer having a smooth surface is typically used which is bonded either directly or indirectly to the core of the roller. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, the belt typically has a smooth, hardened outer surface.
Two basic types of heated roller fusers have evolved. One uses a conformable or compliant pressure roller to form the fusing nip against a hard fuser roller. The other uses a compliant fuser roller to form the nip against a hard or relatively non-conformable pressure roller. A fuser roller designated herein as compliant typically includes a conformable layer having a thickness greater than about 2 mm and in some cases exceeding 25 mm. A fuser roller designated herein as hard includes a rigid cylinder, which may have a relatively thin polymeric or conformable coating, typically less than about 1.25 mm thick. A compliant fuser roller used in conjunction with a hard pressure roller tends to provide easier release of a receiver from the heated fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver towards the relatively non-conformable pressure roller and away from the much more conformable fuser roller.
One common type of fuser roller is internally heated, i.e., a source of heat for fusing is provided within the roller for fusing. Such a fuser roller normally has a hollow core, inside of which is located a heating source, usually a lamp. Surrounding the core is an layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity. A different kind of fuser roller, which is internally heated near its surface, is disclosed by Lee et al. in U.S. Pat. No. 4,791,275, which describes a fuser roller including two polyimide Kapton® sheets (sold by DuPont® and Nemours) having a flexible ohmic heating element disposed between the sheets. The polyimide sheets surround a conformable polyimide foam layer attached to a core member. According to J. H. DuBois and F. W. John, Eds., in Plastics, 5th Edition, Van Nostrand and Rheinhold, 1974, polyimide at room temperature is fairly stiff with a Youngs modulus of about 3.5 GPa-5.5 GPa (1 GPa=1 GigaPascal=10.sup.9 Newton/m.sup.2), but the Young's modulus of the polyimide sheets can be expected to be considerably lower at the stated high operational fusing temperature of the roller of at least 450 degrees F.
Another common type of fuser roller is an externally heated fuser roller. The externally heated fuser roller is heated by surface contact between the fuser roller and one or more external heating rollers. Externally heated fuser rollers are disclosed by O'Leary, U.S. Pat. No. 5,450,183, and by Derimiggio et al., U.S. Pat. No. 4,984,027.
A compliant fuser roller may include a conformable layer of any useful material, such as for example a substantially incompressible elastomer, i.e., having a Poisson's ratio approaching 0.5. A substantially incompressible conformable layer including a poly(dimethyl siloxane) elastomer has been disclosed by Chen et al., in the commonly assigned U.S. Pat. No. 6,224,978, which is hereby incorporated by reference. Alternatively, the conformable layer may include a relatively compressible foam having a value of Poisson's ratio much lower than 0.5. A conformable polyimide foam layer is disclosed by Lee in U.S. Pat. No. 4,791,275 and a lithographic printing blanket are disclosed by Goosen et al. in U.S. Pat. No. 3,983,287, including a conformable layer containing a vast number of frangible rigid-walled tiny bubbles, which are mechanically ruptured to produce a closed cell foam having a smooth surface.
Receivers remove the majority of heat during fusing. Since receivers may have a narrower length measured parallel to the fuser roller axis than the fuser roller length, heat may be removed differentially, causing areas of higher temperature or lower temperature along the fuser roller surface parallel to the roller axis. Higher or lower temperatures can cause excessive toner offset (i.e., toner powder transfer to the fuser roller) in roller fusers.
In the fusing of the toner image to the receiver, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver sheet as it passes through the fusing nip is determined by the amount pressure exerted by the pressure roller and by the characteristics of the resilient conformable layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with, and heated by, the fuser roller.
In a roller fusing system, the fusing parameters, namely the temperature, nip-width, and speed of the fusing member, are fixed and controlled within certain specifications for a given range of receivers. Generally the system changes the temperature or/and speed according to the receiver weights or types. The changing of temperature in an internally heated fuser roller takes time to stabilize. If the receivers are presented at a too-rapid rate, the fuser roller may not have returned to its working temperature when the next receiver arrives. Consequently, the receivers must be stopped or slowed until the temperature of the fuser roller has come within acceptable range and such stopping or slowing results in degradation of receiver throughput rate. The same is true for speed changes. Regardless of whether the speed of presentation or the fuser roller temperature itself is being adjusted by the system, the temperature stabilization time required by a fusing member can constrain the speed of presentation of receivers.
The fixing quality of toned images of an electrophotographic printer depends on the temperature, nip-width, process speed, and thermal properties of the fusing member, toner chemistry, toner coverage, and receiver type. To simplify the engineering and control of a roller fusing system, as many as possible of the above parameters are considered and then fixed during the system's design. The fusing parameters such as temperature, nip-width, process speed, and thermal properties of the fusing member are optimized for the most critical case.
Complicating the systems design is the fact that the toner coverage and the receiver type (weight, coated/uncoated) can vary from image to image in a digital printer. Therefore, some of the above listed parameters need to be adjusted according to the image contents and the receiver types to assure adequate image fixing. Typically, the fuser temperature is adjusted and kept constant for a dedicated run with a particular receiver. The temperature is adjusted higher from the nominal for heavier receivers and lower for lighter receivers. For some heavy receivers, the speed must also be reduced.
The change of fusing speed results in reduced productivity. The change in fusing temperature can also result in reduced productivity because of time spent waiting for the fusing member temperature to change. Furthermore, if different receiver types are required in a single document extra time is needed to collate images on different receivers into the document.
A digital printer with multiple paper supplies allows running RIPPED information that varies from image to image onto multiple receivers in a single document run. Since the RIPPED image may vary from one occurrence to the next both in image color and image density, the workload on the fuser may vary significantly. U.S. Pat. No. 5,956,543, issued to Aslam et al. optimizes the image fixing of toned images on a specified receiver by optimally selecting the fuser temperature, nip-width and speed. However, it does not address the image fixing quality issues when multiple types and weights of receivers are mixed during a document mode operation of an electrophotographic printer.
Another complication with known roller fuser apparatus for high image quality color reproduction involves minimizing gloss variances, while maximizing thermal efficiency to achieve proper application dependent gloss level for the desired reproduction. For achieving high levels of gloss, common control techniques involve maximizing the fuser nip width and the pressure-time relationship of the image-bearing receiver in the fuser nip. In order to provide the proper image quality desired in the market today, image gloss (i.e., luster) control of the fuser has become more important. The ability to match the receiver surface gloss at all image color densities (which implies no differential gloss within a page, or from page to page), as closely as possible, substantially effects and determines the level of image quality with respect to the fusing process operation. The optimal gloss result would be to have no change in gloss within a reproduction page from lead to trail edge, and to have no change in gloss from receiver to receiver in short or long reproduction run jobs.
The fusing surfaces in the fusing nip need to maintain a constant temperature throughout the fusing process to maintain consistent gloss across the entire toner powder image. When a gloss of about 30 G60 units or higher is achieved, gloss variations within the image become more noticeable to the human eye, and the need for improved temperature control is required. Internally heated fuser rollers have a certain time constant for heat to reach the fusing nip surface. The longer the time constant the more difficult it is to maintain a constant fusing temperature, and the temperature range of oscillation increases.
In addition, to attain a high gloss (about 30 G60 units or higher), a relatively large heating (fusing) dwell time is required. Current commercial fusing technology, using low viscosity polyester tones, require a fusing nip dwell time of about 65 milliseconds or greater. Thus, for a 30 page per minute fusing process, the nip width would need to be about 8.5 nm, and for a 60 page per minute fusing process the nip width would need to be about 17.0 mm. To create sc nip widths with rollers, large diameter rollers (2.5 inches to 3.5 inches or larger) with thick elastomer base cushions would be required. Such configuration inherently posses a large thermal mass. Internal heating of the fuser rollers would have a large time constant, and would result in slow heating and difficult temperature control. There would also be significant environmental heating which constitutes substantial wasted energy.